Annular Ring Width

Formula

Description

The annular ring is the width of copper remaining around a drilled hole after drilling. It must be large enough to ensure reliable electrical connection between the copper pad and the barrel plating of the via. IPC-6012 Class 2 (standard commercial) requires a minimum annular ring of 0.05 mm (2 mils) after all manufacturing tolerances. Class 3 (high reliability, aerospace, medical) requires 0.05 mm minimum with additional requirements. Insufficient annular ring can lead to broken connections, intermittent contacts, and reliability failures.

Variables

• AR — Annular ring width (m)
• D_pad — Copper pad diameter (m)
• D_drill — Finished drill hole diameter (m)

Practical Notes

The finished hole diameter is the nominal drill size minus plating thickness (typically 50 µm total). Account for drill registration tolerance (typically ±0.05-0.075 mm). For a Class 2 board with 0.075 mm registration tolerance, the minimum pad diameter is D_drill + 2 × (0.05 + 0.075) = D_drill + 0.25 mm. HDI boards with laser-drilled microvias have tighter registration, allowing smaller annular rings.